(1) Field of the Invention
The present invention is directed to the fabrication of unitary structures of complex shape, such as missile radomes, from fiber reinforced plastic material. More particularly, the present invention relates to hollow structures, such as radomes, which have improved longitudinal strength and, in the case of a radome, improved electromagnetic energy transmission characteristics.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in the manufacture of radomes. Accordingly, the discussion below will be primarily related to radomes.
Ceramic radomes are typically used for missiles intended to operate at speeds of Mach 4 or higher. These ceramic radomes have been found to be at best marginal in performance due to fragility, susceptability to thermal shock, high thermal conductivity and high rates of rain impact damage. A definite need for a workable alternative to ceramic radomes existed for many years.
Radomes made from polymeric materials have been suggested as a possible alternative to ceramic radomes Polytetrafluoroethylene, hereinafter PTFE, is one such polymeric material which might be suitable for radome applications. However, "neat" or simple filled PTFE does not possess the requisite characteristics, uniformity of erosion and ablation for example, for use in the demanding environment of a missile radome. Tests have shown that fiber reinforced PTFE, i.e., a PTFE composite with the fibers having a high aspect ratio, would have those characteristics dictated by radome and similar usage.
Prior to the invention disclosed in application Ser. No. 149,952, now U.S. Pat. No. 4,364,884, it had been a practical impossibility to fabricate a radome from a PTFE-fiber composite. The production of a solid block of PTFE composite of sufficient size to permit machining a radome therefrom is not feasible due to the virtual impossibility of heating such a large block through the crystalline melt point and subsequently cooling through the recrystallization point with enough uniformity of temperature to avoid fissures and damage from thermal stress. Furthermore, even if the temperature gradient and thermal stress problems could be avoided, an extremely long heating and cooling cycle (perhaps on the order of several weeks) would be required, and that long cycle time would result in thermal degradation. Other approaches, such as flowing a sheet of PTFE composite material to form a radome shape or laminating a series of rings or discs cut from such sheet material all involve substantial technical or cost problems which precluded the use of such material and techniques.
My U.S. Pat. No. 4,364,884 discloses a novel radome structure comprised of a fiber reinforced plastic material wherein the fibers are, to a high degree, randomly oriented in planes which are perpendicular to the axis of the radome. This novel fiber reinforced plastic radome is manufactured by sintering together preformed segments of the radome while maintaining axial pressure upon the segments. The preformed segments are formed by cold pressing a powdered PTFE-fiber composite material into rings or discs, the cold pressing step causing the fibers to become oriented randomly in planes perpendicular to the axes of the discs. These discs are machined to form a series of preforms of desired size and shape. The preforms are stacked within a mold cavity and subjected to heat and axial pressure. The resulting unitary sintered structure is machined to form the final desired product.
The final unitary product produced in accordance with the teachings of the above-mentioned U.S. patent overcomes many of the disadvantages of the prior art. It has excellent resistance to ablation and rain erosion and is not as fragile as previous ceramic radomes. Also, a fiber reinforced radome produced in accordance with the teachings of U.S. Pat. No. 4,364,884 is economical to produce when compared to the cost of machining a radome from a large block of PTFE-fiber composite.
However, a radome produced in accordance with the invention of U.S. Pat. No. 4,364,884 possesses. characteristics which limit its usage. For example, since the fibers are oriented in planes perpendicular to the radome axis, the longitudinal tensile strength of the structure is comparatively low. Accordingly, a supporting liner is needed in some cases. The liner will typically be comprised of a glass fiber-epoxy structure or a polyimide-glass fiber honeycomb structure. The bonding of a supporting liner within a previously formed radome may result in the radome fracturing or there may be incomplete bonding between the radome and the supporting liner. The problems associated with bonding a liner within a radome are due in part to the radome having a much higher degree of thermal expansion in the axial direction than does the supporting liner. When fracturing and/or incomplete bonding occurs it will happen during the processing step when heat is applied to cure the adhesive used to bond the liner to the radome. Either voids will form between the liner and the radome due to the radome expansion or the radome will fracture due to tension as it contracts on cooling if there is adequate bonding to the liner. It has also been observed that when exposed to low temperatures the bonded radome and liner assembly experiences axial stresses due to the differences in thermal expansion. These stresses result in tension between the radome and liner which can lead to fissure formation.